This invention relates to compositions that are suitable for use as conductive or resistive materials in microelectronic devices or semiconductor packages to provide electrically stable interconnections.
Conductive and resistive compositions are used for a variety of purposes in the fabrication and assembly of semiconductor packages and microelectronic devices. For example, conductive adhesives are used to bond integrated circuit chips to substrates (die attach adhesives) or circuit assemblies to printed wire boards (surface mount conductive adhesives), and resistive materials are used to form planar or buried resistors in circuit boards.
Two conductors with dissimilar electrochemical potentials will form an electrochemical cell in the presence of water. The conductors act as cathode and anode, and environmental humidity provides the necessary aqueous medium to bridge the anode and cathode. The metal with the higher electrochemical potential acts as the cathode {2H2O+O2+4exe2x86x924OHxe2x88x92}. The metal with the lower electrochemical potential acts as the anode resulting in the loss of electrons {Mxe2x88x92nexe2x86x92Mn+} and the corrosion of the metal. Oxygen is involved in this mechanism but does not directly react with the anode metal. The metal ion Mn+ will combine with OHxe2x88x92 and form a metal hydroxide that stabilizes by developing into a metal oxide, which over time forms on the anode surface. Metal oxides in general are non-conductive, the result being a decrease in conductivity of the metal circuitry.
The problem is less acute when the filler in the composition is the same metal as the contiguous circuitry or the substrate. Thus, a semiconductor package using a conductive composition, one comprising an epoxy resin and silver filler, for example, will not be as susceptible to electrochemical failure when a silver-filled composition is used on a silver substrate. However, if the composition is used on a nickel plated substrate, electrochemical corrosion will result under high humidity conditions. If the composition is a resistive composition containing carbon black as the filler, corrosion will become a problem under high humidity conditions if the substrate contains metal with a low electrochemical potential, such as Ni, Cu, and Sn/Pb solder.
It is known in the art that low melting point metal alloys may improve contact resistance, usually when in combination with certain fluxing agents and cured in a specific manner. For example, U.S. Pat. No. 5,830,389, discloses an electrically conductive composition and its method of preparation and use. The composition of this patent includes a high melting point metal, solder, resin, a reactive monomer or polymer and a chemically protected cross-linking agent with fluxing properties. Similarly, U.S. Pat. No. 5,853,622 discloses a conductive adhesive comprising a high melting point metal which is a substantially spherical powder, a low melting point metal which is also a substantially spherical powder, a chemically protected cross-linking agent, resin, reactive monomer or polymer and a metal additive.
These compositions, however, are vulnerable to environmental conditions, and high temperature and high humidity can cause the electrical resistance of the assembly fabricated with these compositions to increase substantially over time. The suspected mode of failure is electrochemical corrosion of the circuitry at the interface of the conductive filler in the composition with another contiguous metal surface, for example, a metal lead frame or other circuitry.
It is also known in the art that specific corrosion inhibitors may be utilized in electroconductive materials. For example, U.S. Pat. No. 5,951,918 discloses an electroconductive powder or paste which comprises a mixture of silvers resulting in a particular aspect ratio. The electroconductive paste of this patent may optionally contain a corrosion inhibitor such as benzothiazole or benzimidazole. However, there is no disclosure in the art of enhanced contact resistance initially and after environmental aging through a combination of a low melting point alloy and a corrosion inhibitor.
It would be an advantage, therefore, to provide conductive and resistive materials that form electrically stable assemblies for use in semiconductor packaging operations. It would also be advantageous to provide a conductive adhesive which would combine a corrosion inhibitor and a low melting point metal or metal alloy in a manner that would provide improved contact resistance when exposed to harsh environmental conditions.